An olefin polymerization catalyst which is obtained by contacting the following components (A) and (B): component (A): a product of contacting the following components (A.sub.1) and (A.sub.2):component (A.sub.1): polyolefin powder having specific values of pore diameter, pore volume, average pore diameter and average particle diameter,component (A.sub.2): a solid component for a Ziegler catalyst comprising titanium, magnesium and a halogen as the essential components;component (B): an organoaluminum compound.The supporting state of the catalyst is improved, and a contaminant-free polymer having a satisfactorily large particle diameter without the lowering of the activity or the stereoregularity of the catalyst is successfully obtained.